Communication systems in which communication devices mutually communicate through serial communication using photocouplers are known. For example, Patent Literature 1 discloses an air conditioning apparatus of a so-called separate type in which indoor units are connected in parallel to an outdoor unit through a pair of commercial power lines (a power line and a common line) and a signal line. In this air conditioning apparatus, the outdoor unit and the indoor units mutually communicate through serial communication using a current loop that includes the signal line and the common line. Here, each indoor unit is provided with a resistor having a high resistance (approximately a few kΩ to a few tens kΩ) on the current loop for protection of the circuit elements included in each indoor unit in the event that the power supply is improperly connected to the signal line.